Low-heat wireless power receiving device and method

ABSTRACT

The present invention relates to low-heat wireless power receiving device and method for charging a battery with low heat by receiving a wireless power signal from a wireless power transmitting device. When a power receiving coil receives a wireless power signal, a control unit matches impedance by controlling an impedance matching/controlling unit, determines a charging load state of a power receiving unit in accordance with a time-lapse of charging the power receiving unit and a current level detected by a current detecting unit, and selectively turns on a low-heat transforming unit and a high-heat transforming unit in accordance with the selected charging load state, thereby charging the power receiving unit with minimum heat generation.

TECHNICAL FIELD

The present invention relates to a low-heat wireless power receivingdevice configured to charge a battery with low heat by receiving awireless power signal from a wireless power transmitting device, and amethod thereof.

BACKGROUND ART

In general, various portable terminals such as a mobile phone and a PDA(Personal Digital Assistant) are equipped with a power receiving devicesuch as a battery pack charged with a power to supply an operationpower. The power receiving device is charged with a power from anexternal charging device and supplies the power to a portable terminalso that the portable terminal can operate.

The power receiving device may include battery cells charged with apower and a charging/discharging circuit for charging the battery cellswith power from an external charging device and supplying the chargedpower to a portable terminal, and the like.

As a type of connecting the charging device and the power receivingdevice, there has been known a terminal connection type that directlyconnects a power output terminal of a charging device, which receives autility AC power and outputs a power of a voltage and a currentcorresponding to the power receiving device, with a power input terminalof the power receiving device through a cable.

However, according to the terminal connection type, there is a potentialdifference between the terminal of the charging device and the terminalof the power receiving devices, so instantaneous discharge is generatedwhen the terminal of the charging device and the terminal of the powerreceiving device are brought in contact with each other or separatedfrom each other.

The instantaneous discharge wears the terminal of the charging deviceand the terminal of the power receiving device, and when dirt isaccumulated on the terminal of the charging device and the terminal ofthe power receiving device, heat is generated from the dirt and maycause a safety accident such as a fire.

The power stored in the battery cells of the power receiving device isnaturally discharged to the outside through the terminal of the powerreceiving device due to humidity etc., so there is a problem in that thelifespan and performance of the power receiving device are decreased.

Recently, in order to solve the various problems with the terminalconnection type, a wireless power receiving device that charges batterycells by receiving a wireless power signal transmitted from a wirelesspower transmitting device has been proposed (see Korean PatentApplication Publication No. 10-2012-0128114).

The wireless power receiving device is, for example, connected with thewireless power transmitting device in an electromagnetic induction type,receives a wireless power signal wirelessly transmitted from thewireless power transmitting device, and charges battery cells with areceived power.

There have been many efforts for allowing the wireless power receivingdevice to wirelessly receive a power stably and with high efficiency andcharge battery cells.

The wireless power receiving device rectifies a wireless power signalreceived from the wireless power transmitting device with a rectifier toconvert it to a DC power and the voltage level of the DC power isconverted to a voltage level corresponding to the battery cells by aDC/DC converter and then the battery cells are charged with the power.

However, converting the voltage level of the DC power with the DC/DCconverter generates a large amount of heat while the DC/DC converterconverts the voltage level of DC power, so power consumption increasesand there is a need for a specific heat discharge unit for dischargingthe heat generated from the DC/DC converter.

Further, since the heat discharge unit is mounted on a circuit board,the circuit board increases in size.

DISCLOSURE Technical Problem

The present invention provides low-heat wireless power receiving deviceand method capable of reducing heat generated in a process of charging apower receiving device with received power.

Further, the present invention provides a low-heat wireless powerreceiving device including a low-heat transforming unit and a high-heattransforming unit and being capable of reducing a loss of power and heatgeneration by charging a power receiving device with a power whileselectively operating the low-heat transforming unit and the high-heattransforming unit, depending on a charging power of the power receivingdevice, and a low-heat wireless power receiving method.

Technical Solution

According to low-heat wireless power receiving device and method of thepresent invention, a low-heat transforming unit configured to operatewith low heat and supply a charging power to a power receiving unit anda high-heat transforming unit configured to operate with high heat andsupply a stable charging power to the power receiving unit are providedin a charging power output unit.

Further, a control unit determines a charging load of the powerreceiving unit as an initial light load state at an initial stage wherea battery starts to be charged, and turns on the high-heat transformingunit in accordance with the determined initial light load state so thatthe power receiving unit can be supplied with a stable charging power.

Further, when stable charging power is supplied to the power receivingunit in the initial light load state for a predetermined time, a middleload state is determined, and the low-heat transforming unit is turnedon so that a sufficient charging power is supplied to the powerreceiving unit.

In this case, before the low-heat transforming unit is turned on, boththe low-heat transforming unit and the high-heat transforming unit areturned on first while a stable power is supplied to the low-heattransforming unit, and when the low-heat transforming unit has supplieda stable power for a predetermined time, the high-heat transforming unitis turned off, thereby minimizing heat generation.

Further, the control unit determines a charging current of the powerreceiving unit and determines whether the power charging of the powerreceiving unit is in a light load state or in a charged-up state inaccordance with the determined charging current.

When the power charging of the power receiving unit is in the light loadstate, the control unit turns on the high-heat transforming unit so thata charging power can be supplied until the power receiving unit ischarged up.

Further, when it is determined that the power receiving unit has beencharged up, the control unit turns off both the low-heat transformingunit and the high-heat transforming unit and finishes charging.

Therefore, a low-heat wireless power receiving device of the presentinvention includes: a power receiving coil configured to receive awireless power signal; an impedance matching/rectifying unit configuredto match impedance so that the power receiving coil can resonate withthe wireless power signal, and to rectify the wireless power signal toconvert it to a DC power; a charging power supply unit including alow-heat transforming unit and a high-heat transforming unit connectedin parallel with each other and supplying an output power of theimpedance matching/rectifying unit to a power receiving unit as acharging power; a current detecting unit configured to detect a currentlevel of power stored in the power receiving unit; and a control unitconfigured to control an impedance matching of the impedancematching/rectifying unit, to determine a charging load state of thepower receiving unit in accordance with a time-lapse of charging thepower receiving unit and the current level detected by the currentdetecting unit, and to selectively turn on the low-heat transformingunit and the high-heat transforming unit in accordance with thedetermined charging load state.

The control unit determines an initial light load state to turn off thelow-heat transforming unit and to turn on the high-heat transformingunit when the power receiving unit starts to be charged, and determinesa middle load state to turn on the low-heat transforming unit and toturn off the high-heat transforming unit when a predetermined initialcharging time has passed.

Further, the control unit both turns on both the low-heat transformingunit and the high-heat transforming unit for a predetermined time whenthe control unit determines a middle load, and the control unitmaintains the low-heat transforming unit being turned on and turns offthe high-heat transforming unit when the predetermined time has passed.

Further, the control unit determines a light load state to turn off thelow-heat transforming unit and turns on the high-heat transforming unitwhen a detection current level of the current detecting unit enters apredetermined range of a first level.

Further, the control unit both turns on both the low-heat transformingunit and the high-heat transforming unit for a predetermined time whenthe control unit determines the light load state, and the control unitturns off the low-heat transforming unit and turns on the high-heattransforming unit when the predetermined time has passed.

Further, the control unit turns off both the low-heat transforming unitand the high-heat transforming unit when the control unit determinesthat the power receiving unit has been charged up on the basis of adetection current of the current detecting unit.

The low-heat transforming unit may be a transforming unit using an FETas a switching element and the high-heat transforming unit may be anLDO.

Further, a low-heat wireless power receiving method of the presentinvention includes: matching impedance by controlling an impedancematching/controlling unit by means of a control unit when a wirelesspower signal is received by a power receiving coil; determining acharging load state of the power receiving unit in accordance with atime-lapse of charging the power receiving unit and a current leveldetected by a current detecting unit by means of the control unit; andcharging the power receiving unit with a charging power based on thewireless power signal by selectively turning on the low-heattransforming unit and the high-heat transforming unit in accordance withthe determined charging load state by means of the control unit.

The determining of a charging load state of the power receiving unit inaccordance with a time-lapse of charging the power receiving unit and acurrent level detected by a current detecting unit by means of thecontrol unit, may include determining the charging load state of thepower receiving unit as an initial light load state at an initial stagewhere the power receiving unit starts to be charged and determining amiddle load state when a predetermined initial charging time has passed.

The charging of the power receiving unit with a charging power based onthe wireless power signal by selectively turning on the low-heattransforming unit and the high-heat transforming unit in accordance withthe determined charging load state may include: turning off the low-heattransforming unit and turning on the high-heat transforming unit in theinitial light load state; and turning on the low-heat transforming unitand turning off the high-heat transforming unit when the middle load isdetermined.

The turning-on of the low-heat transforming unit and turning-off of thehigh-heat transforming unit when the middle load is determined mayinclude: turning on both the low-heat transforming unit and thehigh-heat transforming unit for a predetermined time; and turning on thelow-heat transforming unit and turning off the high-heat transformingunit when the predetermined time has passed.

The determining of a charging load state of the power receiving unit inaccordance with a time-lapse of charging the power receiving unit and acurrent level detected by a current detecting unit by means of thecontrol unit may include: determining a light load state when adetection current level of the current detecting unit enters apredetermined range of a first level; and determining that the powerreceiving unit has been charged up when the detection current level ofthe current detecting unit is equal to or less than a second level setlower than the first level.

The determining of a light load state when a detection current level ofthe current detecting unit enters a predetermined range of a first levelmay include turning off the low-heat transforming unit and turning onthe high-heat transforming unit by means of the control unit.

The turning-off of the low-heat transforming unit and turning-on of thehigh-heat transforming unit by means of the control unit may include:turning on both the low-heat transforming unit and the high-heattransforming unit for a predetermined time by means of the control unit;and turning off the low-heat transforming unit and keeping the high-heattransforming unit being turned on by means of the control unit when thepredetermined time has passed.

The determining that the power receiving unit has been charged up whenthe detection current level of the current detecting unit is equal to orless than a second level set lower than the first level may include:turning off both the low-heat transforming unit and the high-heattransforming unit by means of the control unit.

Advantageous Effects

The low-heat wireless power receiving device and method of the presentinvention include low-heat transforming unit and a high-heattransforming unit, determines a power charging load state of a powerreceiving unit in accordance with time taken to charging the powerreceiving unit and an output current, and can supply a charging power tothe power receiving unit by selectively turning off the low-heattransforming unit and the high-heat transforming unit in accordance withthe determined power charging load state.

Therefore, it is possible to charge a power receiving unit with asufficient power while minimizing heat generation and to reduce powerconsumption due to low heat generation.

DESCRIPTION OF DRAWINGS

Hereinafter, the present invention will be described with reference tothe accompanying drawings through embodiment not limiting the presentinvention and the same components are given the same reference numbersis some of the drawings.

FIG. 1 is a block diagram illustrating the configuration of a low-heatwireless power receiving device of the present invention.

FIGS. 2 and 3 are signal flowchart illustrating operation of a controlunit according to a low-heat wireless power receiving device of thepresent invention.

FIG. 4 is a table listing example of output voltages of an impedancematching/rectifying unit and charging currents of power receiving deviceaccording to the present invention.

FIG. 5 is a graph illustrating operation by exemplifying output voltagesof an impedance matching/rectifying unit and charging currents of powerreceiving device according to the present invention.

BEST MODE

Hereinafter, low-heat wireless power receiving device and method of thepresent invention will be described in detail with reference to thedrawings.

The suffixes “module” and “unit” used for the components in thefollowing description are provided or used in consideration of onlyconvenience of describing the specification and they do not havedifferentiated meanings or functions by themselves.

FIG. 1 is a block diagram illustrating the configuration of a low-heatwireless power receiving device of the present invention. Referring toFIG. 1, a low-heat wireless power receiving device of the presentinvention may include a power receiving coil 110, an impedancematching/rectifying unit 120, a charging power supply unit 130, acurrent detecting unit 140, a power receiving unit 150 (for example, abattery cell), and a control unit 160.

The power receiving coil 110 is connected with a power transmitting coilof a wireless power transmitting device, for example, an electromagneticinduction type or a magnetic resonance type and receives a wirelesspower signal from the power transmitting coil.

The impedance matching/rectifying unit 120 matches impedance so that thepower receiving coil 110 can resonate with the wireless power signal,and rectifies a wireless power signal received by the power receivingcoil 110 to convert it to a DC power.

The charging power supply unit 130 outputs the DC power, which isoutputted from the impedance matching/rectifying unit 120, as a chargingpower for charging the power receiving unit 150 and may include alow-heat transforming unit 132 and a high-heat transforming unit 134.

The low-heat transforming unit 132 uses a switching element such as anFET (Field Effect Transistor), so when the switching element is turnedon, it passes and outputs the DC power outputted from the impedancematching/rectifying unit 120 as a charging power, and when the switchingelement is turned off, it blocks the DC power outputted from theimpedance matching/rectifying unit 120. The low-heat transforming unit132 does not change the level of the DC power outputted from theimpedance matching/rectifying unit 120, but passes or blocks the DCpower by switching, so it cannot supply a stable DC power to the powerreceiving unit 150, but generates heat very little because it does notreduce the voltage level of the DC power.

The high-heat transforming unit 134 uses, for example, an LDO (Low DropOutput), so when the LDO is turned on, it reduces and outputs the DCpower outputted from the impedance matching/rectifying unit 120 at avoltage level required by the power receiving unit 150, and when the LDOis turned off, it blocks the DC power outputted from the impedancematching/rectifying unit 120. Since the high-heat transforming unit 134outputs a DC power at a voltage level required by the power receivingunit 150, it can supply a stable DC power to the power receiving unit150, but a large amount of heat is generated in the process of downingthe voltage level.

The current detecting unit 140 detects the current of a charging poweroutputted from the low-heat transforming unit 132 or the high-heattransforming unit 134 of the charging power supply unit 130 and storedin the power receiving unit 150.

The control unit 160 controls impedance of the impedancematching/rectifying unit 120 so that the power receiving coil 110 canoptimally receive a wireless power signal, determines a charging loadstate of the power receiving unit 150 in accordance with the chargingtime of the power receiving unit 150 and the detected current of thecurrent detecting unit 140, and controls charging of the power receivingunit 150 by selectively turning on and off the low-heat transformingunit 132 and the high-heat transforming unit 134 in accordance with thedetermined load state.

FIGS. 2 and 3 are signal flowchart illustrating operation of a controlunit according to a low-heat wireless power receiving device of thepresent invention. Referring to FIG. 2, the control unit 160 determineswhether a wireless power signal is received by the impedancematching/rectifying unit 120 (S200). That is when a wireless powertransmitting device transmits a wireless power signal, the powerreceiving coil 110 receives the wireless power signal, the receivedwireless power signal is inputted to the impedance matching/rectifyingunit 120, and the control unit 160 whether the wireless power signal isreceived, by monitoring the impedance matching/rectifying unit 120.

When the wireless power signal is received, as the result ofdetermination, the control unit 160 performs operation of matchingimpedance by controlling the impedance matching/rectifying unit 120(S202).

The operation of matching the impedance of the impedancematching/rectifying unit 120 is made in various ways.

For example, the control unit 160 may match the impedance by receiving asetting value for matching the impedance of the impedancematching/rectifying unit 120 in communication with the wireless powertransmitting device through a communication unit (not illustrated) andcontrolling the impedance matching/rectifying unit 120 in accordancewith the received setting value.

Alternatively, the setting value has a predetermined range and thecontrol unit 160 can match the impedance by detecting the intensity ofthe received wireless power signal while changing the impedance of theimpedance matching/rectifying unit 120 in accordance with the range ofthe setting value and by controlling the impedance matching/rectifyingunit 120 so that the detected intensity of the wireless power signalbecomes the maximum.

Alternatively, the control unit 160 may match the impedance by detectingthe intensity of the received wireless power signal while changing theimpedance of the impedance matching/rectifying unit 120 and bycontrolling the impedance matching/rectifying unit 120 so that thedetected intensity of the wireless power signal becomes the maximum.

Further, the impedance of the impedance matching/rectifying unit 120 canbe matched in various ways other than these ways described above.

In this state, the impedance matching/rectifying unit 120 rectifies awireless power signal inputted from the power receiving coil 100 toconvert it to a DC power and outputs the DC power to the low-heatgenerator 132 and the high-heat generator 134 of the charging powersupply unit 130.

In this case, the control unit 160 does not supply a power to the powerreceiving unit 150, so in which the low-heat transforming unit 132 andthe high-heat transforming unit 134 of the charging power supply unit130 are both turned off.

Accordingly, the voltage level of the power outputted from the impedancematching/rectifying unit 120 is, for example, about 7 to 10.5 V and nocurrent is detected by the current detecting unit 140, as illustrated inFIGS. 4 and 5.

In this state, at the initial stage of charging the power receiving unit150, the control unit 160 determines that the charging load of the powerreceiving unit 150 is in an initial light load state, turns off thelow-heat transforming unit 132 of the charging power supply unit 130,and turns on the high-heat transforming unit 134 (S204).

Then the high-heat transforming unit 134 outputs the voltage level ofthe DC power outputted by the impedance matching/rectifying unit 120through DC/DC conversion and the DC power outputted from the high-heattransforming unit 134 is inputted and stored in the power receiving unit150 through the current detecting unit 140.

Herein, the high-heat transforming unit 134 downs the voltage level ofthe DC power outputted from the impedance matching/rectifying unit 120to the voltage level required by the power receiving unit 150 and thenoutputs it, as described above, so a stable DC power is supplied to thepower receiving unit 150, but high-heat transforming unit 134 changesthe voltage level of the DC power through DC/DC conversion, so itgenerates a large amount of heat.

Further, the high-heat transforming unit 134 outputs a DC power and thepower receiving unit 150 is charged with the DC power, so the voltagelevel of the DC power outputted from the impedance matching/rectifyingunit 120 decreases to, for example, about 5.45 to 5.6 V, as illustratedin FIGS. 4 and 5, and the current detecting unit 140, though dependingon the charging state of the power receiving unit 150, detects a currentof, for example, 200 to 1000 mA when the charging state is low.

Herein, the reason that the control unit 160 charges the power receivingunit 150 by turning on the high-heat transforming unit 134 when it is ainitial light load state, the control unit 160 is for charging the powerreceiving unit 150 with a stable DC power.

In this state, the control unit 160 determines whether the chargingsetting time has passed in the initial light load state.

When the charging setting time has not passed yet, as the result ofdetermination, the control unit 160 determines the current level of thepower stored in the power receiving unit 150 by inputting the detectioncurrent of the current detecting unit 140 (S208) and determines whetherthe charging load of the power receiving unit 150 is in a light loadstate or a charged-up state on the basis of the determined current level(S210 and S212).

That is, when charging is started with the power receiving unit 150charged with a power, the current level of the power stored in the powerreceiving unit 150 decreases in accordance with the amount of anaccumulated power, so, for example when the current level of the powerstored in the power receiving unit 150 is 200 to 500 mA, the controlunit 160 determines that the charging load of the power receiving unit150 is light load, but when it is 80 mA or less, it determines that thepower receiving unit 150 has been charged up.

When the charging load of the power receiving unit 150 is not in a lightload state or a charged-up state, as the result of determination inS208, the control unit 160 returns to S204 and then charges the powerreceiving device 150 with a power by keeping the low-heat transformingunit 132 of the charging power supply unit 130 being turned off andkeeping the high-heat transforming unit 134 being turned on, and repeatsdetermining whether the charging selling time has passed.

In this state, when the charging setting time has passed, the powerreceiving unit 150 has been charged up with a stable initial power andthe control unit 160 determines that the charging load of the powerreceiving unit 150 is in a middle load state.

When the control unit 160 determines that the charging load of the powerreceiving unit 150 is in a middle load state, the control unit 160 turnson both the low-heat transforming unit 132 and the high-heattransforming unit 134 so that the DC power outputted from the impedancematching/rectifying unit 120 passes through the low-heat transformingunit 132 and the high-heat transforming unit 134 and charges the powerreceiving unit 150 through the current detecting unit 140 (S214), andmaintains both the low-heat transforming unit 132 and the high-heattransforming unit 134 being turned on for a predetermined time (S216).

Herein, the reason that the control unit 160 maintains both the low-heattransforming unit 132 and the high-heat transforming unit 134 beingturned on for a predetermined time is for making the high-heattransforming unit 134 keep supplying a charging power until the low-heattransforming unit 132 outputs a charging power.

After maintaining both the low-heat transforming unit 132 and thehigh-heat transforming unit 134 being turned on for a predeterminedtime, the control unit 160 keeps the low-heat transforming unit 132being turned on and turns off the high-heat transforming unit 134(S218).

As the control unit 160 keeps the low-heat transforming unit 132 beingturned on, the power receiving unit 150 keeps being charged and heatgeneration by the charging can be minimized.

In this case, the voltage level of the DC power outputted from theimpedance matching/rectifying unit 120 decreases, for example, to about5.15 to 5.3 V, as illustrated in FIGS. 4 and 5, and the currentdetecting unit 140, though depending on the charging state of the powerreceiving unit 150, detects a current of 350 to 1000 mA.

In this state, the control unit 160 determines the current level of theDC power stored in the power receiving unit 150 by inputting thedetection current of the current detecting unit 140 (S220) anddetermines whether the charging load of the power receiving unit 150 haschanged into a light load state on the basis of the determined currentlevel (S222). That is, the control unit 160 determines whether thecurrent level of the DC power stored in the power receiving unit 150 is,for example, 200 to 500 mA, and when it is 200 to 500 mA, it determinesthat the charging load of the power receiving unit 150 is in a lightload state.

When the control unit 160 determines a light load state in S210 or S222,it turns on both the low-heat transforming unit 132 and the high-heattransforming unit 134 (S224) and maintains both the low-heattransforming unit 132 and the high-heat transforming unit 134 beingturned on for a predetermined time (S226).

Herein, the reason that the control unit 160 maintains both the low-heattransforming unit 132 and the high-heat transforming unit 134 beingturned on for a predetermined time is for making the low-heattransforming unit 132 keeps supplying a charging power until thehigh-heat transforming unit 132 starts to output a stable power.

After maintaining both the low-heat transforming unit 132 and thehigh-heat transforming unit 134 being turned on for a predeterminedtime, the control unit 160 turns off the low-heat transforming unit 132and maintains the high-heat transforming unit 134 being turned on sothat the high-heat transforming unit 134 supplies a charging power tothe power receiving unit 150 (S228).

In this case, as the high-heat transforming unit 134 supplies a chargingpower to the power receiving unit 150, the voltage level of the DC poweroutputted from the impedance matching/rectifying unit 120 increases, forexample, to about 5.45 to 6.0 V, as illustrated in FIGS. 4 and 5, andincreases, for example, to 7.0 to 7.2 V as time passes, and the currentdetecting unit 140, though depending on the charging state of the powerreceiving unit 150, detects a current of about 200 to 500 mA.

In this state, the control unit 160 determines the current level of thepower stored in the power receiving unit 150 by inputting the detectioncurrent of the current detecting unit 140 (S230) and determines whetherthe power receiving unit 150 has been charged up on the basis of thedetermined current level (S232). That is, as illustrated in FIGS. 4 and5, when the power receiving unit 150 is charged up, the voltage level ofthe DC power outputted from the impedance matching/rectifying unit 120increases, for example, to about 7.2 to 7.4 V and the detection currentof the current detecting unit becomes about 80 mA or less, for example.

When the detection current of the current detecting unit 140 is about 80mA or less, the control unit 160 determines that the power receivingunit 150 has been charged up, turns off both the low-heat transformingunit 132 and the high-heat transforming unit 134 (S234), and finishescharging.

The low-heat wireless power receiving device and method described aboveis not limited to the configuration and method of the embodimentsdescribed above, and all or some of the embodiments may be selectivelycombined so that the embodiments can be modified in various ways.

The invention claimed is:
 1. A low-heat wireless power receiving device comprising: a power receiving coil configured to receive a wireless power signal; an impedance matching/rectifying unit configured to match impedance so that the power receiving coil can resonate with the wireless power signal and to rectify the wireless power signal to convert it to a DC power; a charging power supply unit including a low-heat transforming unit and a high-heat transforming unit connected in parallel with each other and supplying an output power of the impedance matching/rectifying unit to a power receiving unit as a charging power; a current detecting unit configured to detect a current level of a power stored in the power receiving unit; and a control unit configured to control an impedance matching of the impedance matching/rectifying unit, to determine a charging load state of the power receiving unit in accordance with a time-lapse of charging the power receiving unit and the current level detected by the current detecting unit, and to selectively turn on the low-heat transforming unit and the high-heat transforming unit in accordance with the determined charging load state.
 2. The low-heat wireless power receiving device of claim 1, wherein the control unit determines an initial light load state to turn off the low-heat transforming unit and to turn on the high-heat transforming unit when the power receiving unit starts to be charged, and determines a middle load state to turn on the low-heat transforming unit and to tum off the high-heat transforming unit when a predetermined initial charging time has passed.
 3. The low-heat wireless power receiving device of claim 2, wherein the control unit turns on both the low-heat transforming unit and the high-heat transforming unit for a predetermined time when the control unit determines a middle load, and the control unit maintains the low-heat transforming unit being turned on and turns off the high-heat transforming unit when the predetermined time has passed.
 4. The low-heat wireless power receiving device of claim 1, wherein the control unit determines a light load state to turn off the low-heat transforming unit and turns on the high-heat transforming unit when a detection current level of the current detecting unit enters a predetermined range of a first level.
 5. The low-heat wireless power receiving device of claim 4, wherein the control unit turns on both the low-heat transforming unit and the high-heat transforming unit for a predetermined time when the control unit determines the light load state, and the control unit turns off the low-heat transforming unit and turns on the high-heat transforming unit when the predetermined time has passed.
 6. The low-heat wireless power receiving device of claim 1, wherein the control unit turns off both the low-heat transforming unit and the high-heat transforming unit when the control unit determines that the power receiving unit has been charged up on the basis of a detection current of the current detecting unit.
 7. The low-heat wireless power receiving device of claim 1, wherein the low-heat transforming unit is a transforming unit using an FET as a switching element and the high-heat transforming unit is an LDO.
 8. A low-heat wireless power receiving method comprising: matching impedance by controlling an impedance matching/controlling unit by means of a control unit when a wireless power signal is received by a power receiving coil; determining a charging load state of a power receiving unit in accordance with a time-lapse of charging the power receiving unit and a current level detected by a current detecting unit by means of the control unit; and charging the power receiving unit with a charging power based on the wireless power signal by selectively turning on a low-heat transforming unit and a high-heat transforming unit in accordance with the determined charging load state by means of the control unit.
 9. The method of claim 8, wherein the determining of a charging load state of the power receiving unit in accordance with a time-lapse of charging the power receiving unit and a current level detected by a current detecting unit by means of the control unit, includes: determining the charging load state of the power receiving unit as an initial light load state at an initial stage where the power receiving unit starts to be charged and determining a middle load state when a predetermined initial charging time has passed.
 10. The method of claim 9, wherein the charging of the power receiving unit with a charging power based on the wireless power signal by selectively turning on the low-heat transforming unit and the high-heat transforming unit in accordance with the determined charging load state by means of the control unit, includes: turning off the low-heat transforming unit and turning on the high-heat transforming unit in the initial light load state; and turning on the low-heat transforming unit and turning off the high-heat transforming unit when the middle load is determined.
 11. The method of claim 10, wherein the turning-on of the low-heat transforming unit and turning-off of the high-heat transforming unit when the middle load is determined, includes: turning on both the low-heat transforming unit and the high-heat transforming unit for a predetermined time; and turning on the low-heat transforming unit and turning off the high-heat transforming unit when the predetermined time has passed.
 12. The method of claim 8, wherein the determining of a charging load state of the power receiving unit in accordance with a time-lapse of charging the power receiving unit and a current level detected by a current detecting unit by means of the control unit, includes: determining a light load state when a detection current level of the current detecting unit enters a predetermined range of a first level; and determining that the power receiving unit has been charged up when the detection current level of the current detecting unit is equal to or less than a second level set lower than the first level.
 13. The method of claim 12, wherein the determining of a light load state when a detection current level of the current detecting unit enters a predetermined range of a first level includes: turning off the low-heat transforming unit and turning on the high-heat transforming unit by means of the control unit.
 14. The method of claim 13, wherein the turning-off of the low-heat transforming unit and turning-on of the high-heat transforming unit by means of the control unit, includes: turning on both the low-heat transforming unit and the high-heat transforming for a predetermined time by means of the control unit; and turning off the low-heat transforming unit and keeping the high-heat transforming unit being turned on by means of the control unit when the predetermined time has passed.
 15. The method of claim 12, wherein the determining that the power receiving unit has been charged up when the detection current level of the current detecting unit is equal to or less than a second level set lower than the first level includes: turning off both the low-heat transforming unit and the high-heat transforming unit by means of the control unit. 